Conventional trailers come in two basic design configurations. The first, a development of the original two axle horse-drawn cart, has a front axle which pivots about a vertical axis, and a drawbar which is connected to the axle assembly so that the rear of the drawbar pivots about a horizontal transverse axis. The full weight of the trailer is accordingly supported by its front and rear axles and, if the trailer is braked, the only significant hitch loads at the rear of the towing vehicle are those required to pull the trailer. However, the inherent weakness of this design of trailer is that it has two articulation points: one at the hitch point at the rear of the towing vehicle, and the second at the vertical pivot axis of the front axle. Various problems arise from the presence of two articulation points. First, the drawbar tends to jackknife when the vehicle combination is operated in reverse, so that it is impractical to manoeuvre such a trailer in reverse except for short distances in a straight line. Secondly, the drawbar tends to jackknife under emergency braking. Thirdly, the trailer suffers from the phenomenon known as rearward amplification. That is to say, the trailer swings more than the towing vehicle during high speed steering manoeuvres. This is a particular problem when such several trailers are connected together.
The second basic type of trailer has most of its weight supported by one or more centrally positioned axles. The axles are centrally positioned in the sense that they are approximately under the centre of gravity of the trailer. A rigid drawbar, which is usually part of the trailer frame, extends forward and is fitted with a hitch for connection to the rear of the towing vehicle. The advantage of this design is that it has only one articulation point, at the hitch point at the rear of the towing vehicle, and so it is relatively easy to manoeuvre such a trailer in reverse. The disadvantage of this design is that, in order to achieve dynamic stability at highway speeds, a significant portion of the trailer weight has to be supported at the hitch point. This extra weight at the rear of the towing vehicle adversely affects its dynamic stability at highway speeds and can result in overloading of its rear axle unless some type of load-levelling hitch is employed. In addition, the towing vehicle and the trailer are coupled in pitch and yaw (vertical rotation about the vertical axis). As a result pitching motion and trailer swing are immediately transmitted from the trailer to the towing vehicle, which adversely affects both the stability of the towing vehicle and the comfort of its occupants.